A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
A player, 20 January 2011
while looking at the mouth of the plague, i noticed that it did not say anything about current battle field or anything like other regions. are regions exempt from the rules of presence? if opposing the mouth of the plague, and i attack adjacent to the mouth of the plague, can i targeted by the -1c tokens?
Mouth of the Plague Region Fire • Mountain • Plague This province does not hold Dynasty cards unless you are a Crane Clan player. Battle: Give -1C Plague token to one or two target Personalities
A player, 20 January 2011
Mouth of the Plague is not "a card in a unit" so the Rule of Location does not forbid its player to use its Battle ability whereas this Region is not attached to the current battlefield's province. In your example, your Personalities at the current battlefield (that is adjacent to the battlefield whose province has Mouth of the Plague attached) could be legally targeted by the Battle action from this Region.